Epoxy resins are excellent in electrical insulating property, heat resistance, chemical resistance, and workability, and therefore are used as, for example, semiconductor encapsulating materials, laminates for printed circuit boards, insulating varnishes, and the like particularly in electric and electronic fields. Solventless type varnishes containing epoxy resins and acid anhydride hardeners are generally used as insulation casting resins for electric and electronic instruments. In particular, epoxy resins are used for casting for large-sized electric parts for heavy electric instruments and large-sized structures, which are required to have mechanical strength and toughness. Bisphenol A diglycidyl ether type epoxy resins having an epoxy equivalent of 250 to 500 g/equivalent are used in combination with acid anhydride hardeners.
In the case of large-sized cast products, it is necessary to conduct curing reaction slowly over a long period of time and to make exotherm due to curing reaction gentle in order to decrease stress due to curing shrinkage and residual heat stress after curing. For this reason, accelerators are not added to the formulation of epoxy resins and acid anhydride hardeners, or if added, accelerators are added to the formulation in very slight amount. As the size of cast products increases, curing reaction is conducted mildly, and control of curing process such as temperature and time in the course of curing is conducted very strictly.
However, conventional casting epoxy resins have problems that, in particular, in the formulation of the resins and acid anhydride hardeners, where accelerators are not added to the formulation, even if control of temperature and time in the course of curing is strictly conducted, the properties of obtained cast products such as heat resistance or crack resistance vary widely. Therefore, for the epoxy resins used in casting, curing reactivity of the resins and properties of cured products are previously confirmed prior to use thereof, and only specific production lot is selected and used. Further, depending on the results by such a confirmation, formulation and curing conditions may be adjusted. However, this leads to complicated process control.
Therefore, it is desirable to obtain an epoxy resin composition useful in casting applications, in which the cured cast products finally obtained are excellent in heat resistance and crack resistance, and have stable qualities.